Solid laser apparatus

ABSTRACT

A solid laser apparatus has either a combination of a lowpass filter and a highpass filter or a bandpass filter provided in its output control circuit for increasing the gain and adjusting the phase of a signal in the vicinity of the relaxation oscillating frequency. Also, a solid laser apparatus has the phase shift circuit provided in its output control circuit for advancing the phase of a signal in the vicinity of the relaxation oscillating frequency thereof. Moreover, a solid laser apparatus has a pseudo notch filter, which is arranged for the gain to have a local minimum at the relaxation oscillating frequency of the solid laser apparatus, provided in the output control circuit so that the peak of the gain transmission characteristic of the nonlinear optical device or of a combination of the nonlinear optical device and the microchip laser crystal is offset.

FIELD OF THE INVENTION

[0001] The present invention relates to a solid laser apparatus and more particularly to a solid laser apparatus which can effectively attenuate optical noises.

BACKGROUND OF THE INVENTION

[0002] A solid laser apparatus is known comprising a semiconductor laser for generating a laser beam, a nonlinear optical device for receiving the laser beam from the semiconductor laser and emitting its harmonic light, an optical detecting means used as a monitor for detecting the intensity of the light emitted from the nonlinear optical device, and an output control circuit for driving the semiconductor laser so as to maintain the intensity of the light at a predetermined level (See Citation 1).

[0003] Another solid laser apparatus is known having a microchip laser crystal designed for being excited by the laser beam emitted from a semiconductor laser, provided with its crystal end faces coated for use as an optical resonator, and located at the upstream of an nonlinear optical device (See Citation 2).

[0004] Citation 1 disclosed in Japanese Patent Laid-open Publication (Heisei) 7-106682.

[0005] Citation 2 disclosed in Japanese Patent Laid-open Publication (Heisei) 4-503429.

[0006] (Problems that the Invention is to Solve)

[0007]FIG. 20 illustrates an actual measurement profile of gain transmission characteristic and phase transmission characteristic of the nonlinear optical device and the microchip laser crystal in such a conventional solid laser apparatus.

[0008] As shown in FIG. 20, the gain peak of a signal appears about 11 MHz. The frequency at the gain peak is referred to as a relaxation oscillating frequency fk of the solid laser apparatus. Also, the profile of FIG. 20 illustrates the phase of the signal is inverted around the relaxation oscillating frequency fk and retarded by substantially 90 degrees in the vicinity of the relaxation oscillating frequency fk.

[0009]FIG. 21 illustrates a measurement profile of optical noise waveforms in the solid laser apparatus.

[0010] The profile of FIG. 21 illustrates the oscillating frequency of optical noise at substantially 11 MHz which is equal to the relaxation oscillating frequency fk shown in FIG. 20.

[0011] The solid laser apparatus having the phase transmission characteristic shown in FIG. 20 allows a range of frequencies lower than the relaxation oscillating frequency fk to be successfully controlled by the negative feedback controlling action of the output control circuit.

[0012] However, if a range of frequencies higher than the relaxation oscillating frequency fk contains any external interruption such as a circuitry noise due to the inversion of the phase transmission characteristic in the vicinity of the relaxation oscillating frequency fk as shown in FIG. 20, it may cause the control system to start oscillation and disturb the negative feedback controlling action thus failing to effectively attenuate the optical noise. Also, if an external interruption such as the circuitry noise is contained in the range of frequencies lower than the relaxation oscillating frequency fk, it may be treated as an optical noise thus allowing the attenuation of optical noise with much difficulty. Moreover, if the optical noise in the vicinity of the relaxation oscillating frequency fk is delayed by substantially 90 degrees in the phase, it may be attenuated only to a limited level by the negative feedback controlling action of the output control circuit.

[0013] On the other hand, FIG. 22 illustrates a profile of gain transmission characteristic of the nonlinear optical device and the microchip laser crystal in such a conventional solid laser apparatus.

[0014] As shown in FIG. 22, the relaxation oscillating frequency fk of a solid laser apparatus appears about 12 MHz.

[0015]FIG. 23 illustrates a profile of phase transmission characteristic of the nonlinear optical device and the microchip laser crystal in the conventional solid laser apparatus.

[0016] As shown in FIG. 23, the phase of the signal is inverted in the vicinity of the relaxation oscillating frequency fk.

[0017] The solid laser apparatus having the phase transmission characteristic shown in FIG. 23 allows any interruption (low frequency noise) in the output at frequencies lower than the relaxation oscillating frequency fk to be successfully controlled by the negative feedback controlling action of the output control circuit. This is because the common operating frequencies of the solid laser apparatus are sufficiently lower than the relaxation oscillating frequency fk.

[0018] However, since the phase transmission characteristic is inverted at the relaxation oscillating frequency fk as shown in FIG. 23, the negative feedback action will be turned to a positive feedback action hence failing to suppress an interruption (high frequency noise) in the output at the frequencies higher than the relaxation oscillating frequency fk.

[0019] It is hence an object of the present invention to provide a solid laser apparatus which can effectively attenuate optical noises.

SUMMARY OF THE INVENTION

[0020] According to a first aspect of the present invention, a solid laser apparatus having a semiconductor laser for emitting a laser beam, a microchip laser crystal arranged for being excited by the laser beam emitted from the semiconductor laser and provided with its crystal end faces coated for use as an optical resonator, a nonlinear optical device for receiving a laser beam from the microchip laser crystal and emitting its harmonic light, an optical detecting means used as a monitor for detecting the intensity of the light emitted from the nonlinear optical device, and an output control circuit for driving the semiconductor laser so as to maintain the intensity of the light at a predetermined level is provided wherein the output control circuit includes either a combination of a lowpass filter and a highpass filter or a bandpass filter.

[0021] Such an optical noise may be generated in the vicinity of the relaxation oscillating frequency fk and remain delayed by substantially 90 degrees in the phase.

[0022] Therefore, the optical noise can effectively be attenuated by extracting a target range of the signal in the vicinity of the relaxation oscillating frequency fk and controlling its phase.

[0023] The solid laser apparatus of the first aspect includes either a combination of the lowpass filter and the highpass filter or the bandpass filter for minimizing the decay ratio of the signal in the vicinity of the relaxation oscillating frequency fk.

[0024] The transmission characteristic by a combination of the lowpass filter and the highpass filter or the bandpass filter allows the decay ratio of the signal to remain high throughout the range of frequencies other than the relaxation oscillating frequency fk. Accordingly, as the external interruption (e.g. circuitry noise) is removed effectively, the range of the signal where the optical noise is carried can be extracted thus effecting the attenuation of the optical noise.

[0025] Moreover, the phase of a signal can be advanced in the vicinity of the relaxation oscillating frequency fk by having the cutoff frequency of the highpass filter set higher than the cutoff frequency of the lowpass filter or adjusting the center frequency of the bandpass filter. Accordingly, any phase delay of the optical noise produced in the vicinity of the relaxation oscillating frequency fk can favorably be corrected.

[0026] In short, the combination of the lowpass filter and the highpass filter or the bandpass filter allows a target range of the signal where the optical noise is carried to be effectively extracted and subjected to the phase compensation, hence effecting the attenuation of the optical noise through the feedback controlling action of the output control circuit.

[0027] According to a second aspect of the present invention, a solid laser apparatus having a semiconductor laser for emitting a laser beam, a nonlinear optical device for receiving a laser beam from the semiconductor laser and emitting its harmonic light, an optical detecting means used as a monitor for detecting the intensity of the light emitted from the nonlinear optical device, and an output control circuit for driving the semiconductor laser so as to maintain the intensity of the light at a predetermined level, wherein the output control circuit includes either a combination of a lowpass filter and a highpass filter or a bandpass filter.

[0028] Such an optical noise may be generated in the vicinity of the relaxation oscillating frequency fk and delayed by substantially 90 degrees in the phase.

[0029] Therefore, the optical noise can effectively be attenuated by extracting a target range of the signal in the vicinity of the relaxation oscillating frequency fk and controlling its phase.

[0030] The solid laser apparatus of the second aspect also includes either a combination of the lowpass filter and the highpass filter or the bandpass filter for minimizing the decay ratio of the signal in the vicinity of the relaxation oscillating frequency fk.

[0031] The transmission characteristic by the combination of the lowpass filter and the highpass filter or the bandpass filter allows the decay ratio of the signal to remain high throughout the range of frequencies other than the relaxation oscillating frequency fk. Accordingly, as the external interruption (e.g. circuitry noise) is removed effectively, the range of the signal where the optical noise is carried can be extracted thus effecting the attenuation of the optical noise.

[0032] Moreover, the phase can be advanced in the vicinity of the relaxation oscillating frequency fk by having the cutoff frequency of the highpass filter set higher than the cutoff frequency of the lowpass filter or adjusting the center frequency of the bandpass filter. Accordingly, any phase delay of the optical noise produced in the vicinity of the relaxation oscillating frequency fk can favorably be corrected.

[0033] In brief, the combination of the lowpass filter and the highpass filter or the bandpass filter allows a target range of the signal where the optical noise is carried to be effectively extracted and subjected to the phase compensation, hence effecting the attenuation of the optical noise through the feedback controlling action of the output control circuit.

[0034] According to a third aspect of the present invention, a solid laser apparatus having a semiconductor laser for emitting a laser beam, a microchip laser crystal arranged for being excited by the laser beam emitted from the semiconductor laser and provided with its crystal end faces coated for use as an optical resonator, a nonlinear optical device for receiving a laser beam from the microchip laser crystal and emitting its harmonic light, an optical detecting means used as a monitor for detecting the intensity of the light emitted from the nonlinear optical device, and an output control circuit for driving the semiconductor laser so as to maintain the intensity of the light at a predetermined level is provided wherein the output control circuit includes a phase shift circuit for advancing the phase of a signal in the vicinity of the relaxation oscillating frequency of the solid laser apparatus.

[0035] The solid laser apparatus of the third aspect has the phase shift circuit provided for advancing the phase of a signal in the vicinity of the relaxation oscillating frequency thereof. The phase shift characteristic of the phase shift circuit in combination with the phase transmitting characteristic of the nonlinear optical device and the microchip laser crystal then permits the delay in the phase of the signal in the vicinity of the relaxation oscillating frequency to be reduced to almost zero degree, hence enabling the attenuation of any optical noise through the feedback controlling action of the output control circuit.

[0036] According to a fourth aspect of the present invention, a solid laser apparatus having a semiconductor laser for emitting a laser beam, a nonlinear optical device for receiving a laser beam from the semiconductor laser and emitting its harmonic light, an optical detecting means used as a monitor for detecting the intensity of the light emitted from the nonlinear optical device, and an output control circuit for driving the semiconductor laser so as to maintain the intensity of the light at a predetermined level is provided wherein the output control circuit includes a phase shift circuit for advancing the phase of a signal in the vicinity of the relaxation oscillating frequency of the solid laser apparatus.

[0037] The solid laser apparatus of the fourth aspect has the phase shift circuit provided for advancing the phase of a signal in the vicinity of the relaxation oscillating frequency thereof. The phase shift characteristic of the phase shift circuit in combination with the phase transmitting characteristic of the nonlinear optical device then permits the delay in the phase of the signal in the vicinity of the relaxation oscillating frequency to be reduced to almost zero degree, hence enabling the attenuation of any optical noise through the feedback controlling action of the output control circuit.

[0038] According to a fifth aspect of the present invention, a solid laser apparatus having a semiconductor laser for emitting a laser beam, a microchip laser crystal arranged for being excited by the laser beam emitted from the semiconductor laser and provided with its crystal end faces coated for use as an optical resonator, a nonlinear optical device for receiving a laser beam from the microchip laser crystal and emitting its harmonic light, an optical detecting means used as a monitor for detecting the intensity of the light emitted from the nonlinear optical device, and an output control circuit for driving the semiconductor laser so as to maintain the intensity of the light at a predetermined level is provided wherein the output control circuit includes a pseudo notch filter of which gain is arranged to have a local minimum at the relaxation oscillating frequency and not zero at the notch frequency.

[0039] The pseudo notch filter, but not an ideal notch filter for minimizing the gain to zero in the vicinity of the relaxation oscillating frequency of the solid laser apparatus, is provided in the above arrangement because its gain characteristic is assumed for gradually decreasing the gain to a local minimum but not zero in the vicinity of the relaxation oscillating frequency.

[0040] The solid laser apparatus according to the fifth aspect has the pseudo notch filter arranged for providing a local minimum of the gain in the vicinity of the relaxation oscillating frequency thereof. The gain transmission characteristic of the pseudo notch filter enables to offset the peak of the gain transmission characteristic of the nonlinear optical device and the microchip laser crystal and can thus favorably effect the attenuation of optical noise in the vicinity of the relaxation oscillating frequency. Also, since the phase transmission characteristic of the pseudo notch filter provides inversion of the phase in the vicinity of the relaxation oscillating frequency, its action in combination with the phase transmission characteristic of the nonlinear optical device and the microchip laser crystal allows the phase to remain not inverted in the vicinity of the relaxation oscillating frequency, hence enabling the attenuation of both low frequency noise and high frequency noise through the feedback controlling action of the output control circuit.

[0041] According to a sixth aspect of the present invention, a solid laser apparatus having a semiconductor laser for emitting a laser beam, a nonlinear optical device for receiving a laser beam from the semiconductor laser and emitting its harmonic light, an optical detecting means used as a monitor for detecting the intensity of the light emitted from the nonlinear optical device, and an output control circuit for driving the semiconductor laser so as to maintain the intensity of the light at a predetermined level is provided wherein the output control circuit includes a pseudo notch filter of which gain is arranged to have a local minimum at the relaxation oscillating frequency and not zero at the notch frequency.

[0042] The pseudo notch filter, but not an ideal notch filter for minimizing the gain to zero in the vicinity of the relaxation oscillating frequency of the solid laser apparatus, is provided in the above arrangement because its gain characteristic is assumed for gradually decreasing the gain to a local minimum but not zero in the vicinity of the relaxation oscillating frequency.

[0043] The solid laser apparatus according to the sixth aspect has the pseudo notch filter arranged for providing a local minimum of the gain in the vicinity of the relaxation oscillating frequency thereof. The gain transmission characteristic of the pseudo notch filter enables to offset the peak of the gain transmission characteristic of the nonlinear optical device and can thus favorably effect the attenuation of optical noise in the vicinity of the relaxation oscillating frequency. Also, since the phase transmission characteristic of the pseudo notch filter provides inversion of the phase in the vicinity of the relaxation oscillating frequency, its action in combination with the phase transmission characteristic of the nonlinear optical device allows the phase to remain not inverted in the vicinity of the relaxation oscillating frequency, hence enabling the attenuation of both low frequency noise and high frequency noise through the feedback controlling action of the output control circuit.

ADVANTAGE OF THE INVENTION

[0044] As set forth above, the solid laser apparatus according to the present invention has either the combination of the lowpass filter and the highpass filter or the bandpass filter provided in the output control circuit for increasing the gain in the vicinity of the relaxation oscillating frequency and carrying out the phase controlling action in the vicinity of the relaxation oscillating frequency. Since the gain becomes low throughout the range of frequencies other than the relaxation oscillating frequency, any external interruption (e.g. circuitry noise) can effectively be rejected thus effecting the attenuation of optical noise.

[0045] Also, the solid laser apparatus according to the present invention has the phase shift circuit provided in its output control circuit for advancing the phase of a signal in the vicinity of the relaxation oscillating frequency thereof. This allows the phase shift action of the phase shift circuit to be operative, thus effecting the attenuation of the optical noise through the feedback controlling action of the output control circuit.

[0046] Moerover, the solid laser apparatus according to the present invention has a pseudo notch filter, which is arranged for the gain to have a local minimum at the relaxation oscillating frequency of the solid laser apparatus, provided in the output control circuit so that the peak of the gain transmission characteristic of the nonlinear optical device or of a combination of the nonlinear optical device and the microchip laser crystal is offset, hence effecting the attenuation of optical noise in the vicinity of the relaxation oscillating frequency. Since the phase transmission characteristic of the pseudo notch filter is inverted in the vicinity of the relaxation oscillating frequency, its action in combination with the phase transmission characteristic of the nonlinear optical device or of the nonlinear optical device and the microchip laser crystal, which provides inversion of the phase in the vicinity of the relaxation oscillating frequency, permits the phase to remain not inverted in the vicinity of the relaxation oscillating frequency. Accordingly, both low frequency noise and high frequency noise can effectively be attenuated by the feedback controlling action of the output control circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047]FIG. 1 is a schematic view of a solid laser apparatus showing the first embodiment of the present invention;

[0048]FIG. 2 is a circuitry diagram showing an exemplary circuit of the lowpass filter;

[0049]FIG. 3 is a circuitry diagram showing an exemplary circuit of the highpass filter;

[0050]FIG. 4 is a circuitry diagram showing an exemplary circuit of the bandpass filter;

[0051]FIG. 5 is a profile of grain transmission function in the lowpass filter;

[0052]FIG. 6 is a profile of phase transmission function in the lowpass filter;

[0053]FIG. 7 is a profile of grain transmission function in the highpass filter;

[0054]FIG. 8 is a profile of phase transmission function in the highpass filter;

[0055]FIG. 9 is a profile of gain transmission function in the bandpass filter;

[0056]FIG. 10 is a profile of phase transmission function in the bandpass filter;

[0057]FIG. 11 is a schematic view of a solid laser apparatus showing the second embodiment of the present invention;

[0058]FIG. 12 is a schematic view of a solid laser apparatus showing the third embodiment of the present invention;

[0059]FIG. 13 is a circuitry diagram showing an exemplary circuit of the phase shift circuit;

[0060]FIG. 14 is a schematic view of a solid laser apparatus showing the fourth embodiment of the present invention;

[0061]FIG. 15 is a schematic view of a solid laser apparatus showing the fifth embodiment of the present invention;

[0062]FIG. 16 is a circuitry diagram showing an exemplary circuit of the pseudo notch filter;

[0063]FIG. 17 is a profile of grain transmission function in the pseudo notch filter;

[0064]FIG. 18 is a profile of phase transmission function in the pseudo notch filter;

[0065]FIG. 19 is a schematic view of a solid laser apparatus showing the sixth embodiment of the present invention;

[0066]FIG. 20 is an actual measurement profile of the gain transmission characteristic and the phase transmission characteristic of a nonlinear optical device or a microchip laser crystal in a conventional solid laser apparatus;

[0067]FIG. 21 is an actual measurement profile of optical noise waveform in the conventional solid laser apparatus;

[0068]FIG. 22 is a profile of the gain transmission characteristic of a nonlinear optical device or of a nonlinear optical device and a microchip laser crystal in a conventional solid laser apparatus; and

[0069]FIG. 23 is a profile of the phase transmission characteristic of the nonlinear optical device or of the nonlinear optical device and the microchip laser crystal in the conventional solid laser apparatus, in which the primary components are

[0070]1. Semiconductor laser, 3. Microchip laser crystal, 4. Non-linear optical device, 8. Low speed APC circuit, 8 c. Lowpass filter, 9. High speed APC circuit, 9 c. Lowpass filter, 9 d. Highpass filter, 9 f. Bandpass filter, 9 g. Phase shift circuit, 9 h. Pseudo notch filter, 100, 200, 300, 400, 500, 600. Solid laser apparatuses.

BEST MODES FOR EMBODYING THE INVENTION

[0071] Preferred embodiments of the present invention will now be described referring to the relevant drawings. It would be understood that the present invention is not limited to the described embodiments.

[0072] (First Embodiment)

[0073]FIG. 1 is a schematic diagram of a solid laser apparatus 100 showing the first embodiment of the present invention.

[0074] The solid laser apparatus 100 comprises a semiconductor laser 1 for generating a laser beam, a collective lens system 2 for focusing the laser beam, a microchip laser crystal 3 provided with its crystal end faces coated for use as an optical resonator which can be excited by the focused laser beam, a nonlinear optical device 4 for receiving the laser beam from the microchip laser crystal 3 and emitting its harmonic light, an assembly of a splitter 5, an optical filter 6, and a photo diode 7 for detecting the intensity of the light emitted from the nonlinear optical device 4, a low speed APC (automatic power control) circuit 8 for releasing a control signal L to set the intensity of the beam detected by the photo diode 7 to a predetermined level, a high speed APC circuit 9 for releasing a control signal H to attenuate noise components in the beam detected by the photo diode 7 to zero, and an LD drive circuit 10 for supplying the semiconductor laser 1 with a driving current determined by the control signals L and H.

[0075] The low speed APC circuit 8 includes a signal amplifier circuit 8 b, a lowpass filter 8 c, and a signal inverse amplifier circuit 8 e.

[0076] The high speed APC circuit 9 includes a coupling capacitor 9 a, a signal amplifier circuit 9 b, a lowpass filter 9 c, a highpass filter 9 d, and a signal inverse amplifier circuit 9 e. The lowpass filter 9 c and the highpass filter 9 d may be replaced by a bandpass filter 9 f.

[0077]FIG. 2 illustrates an exemplary circuit of each of the lowpass filters 8 c and 9 c.

[0078] The circuit of FIG. 2 may be either (a) or (b).

[0079]FIG. 3 illustrates an exemplary circuit of the highpass filter 9 d.

[0080] The circuit of FIG. 3 may be either (a) or (b).

[0081]FIG. 4 illustrates an exemplary circuit of the bandpass filter 9 f.

[0082] The circuit of FIG. 2 may be either (a) or (b).

[0083]FIG. 5 illustrates a profile of gain transmission characteristic in the lowpass filters 8 c and 9 c.

[0084]FIG. 7 illustrates a profile of gain transmission characteristic in the highpass filter 9 d.

[0085] Since the transmission characteristic in a combination of the lowpass filter 9 c and the highpass filter 9 d allows the gain to be declined throughout the range of frequencies other than the relaxation oscillating frequency fk, it can effectively reject any external interruption (e.g. circuitry noise) thus effecting the attenuation of optical noise.

[0086]FIG. 6 illustrates a profile of phase transmission characteristic in each of the lowpass filters 8 c and 9 c.

[0087]FIG. 8 illustrates a profile of phase transmission characteristic in the highpass filter 9 d.

[0088] Since the cutoff frequency of the highpass filter 9 d is determined higher than the cutoff frequency of the lowpass filter 9 c, the phase at the relaxation oscillating frequency fk can be advanced hence permitting any phase delay of the optical noise produced in the vicinity of the relaxation oscillating frequency fk to be corrected.

[0089] The combination of the lowpass filter 9 c and the highpass filter 9 d permits a range of the signal where the optical noise is carried to be extracted and subjected to the phase compensation and can hence effectively attenuate the optical noise through the feedback controlling action of the high speed APC circuit 9.

[0090] The degree of each of the lowpass filter 9 c and the highpass filter 9 d can arbitrarily be determined.

[0091]FIG. 9 illustrates a profile of gain transmission characteristic in the bandpass filter 9 f.

[0092]FIG. 10 illustrates a profile of phase transmission characteristic in the bandpass filter 9 f.

[0093] Since both the center frequency and the degree of the bandpass filter 9 f is desirably determined, they can allows a range of the signal where the optical noise is carried to be extracted and subjected to the phase compensation, hence effecting the attenuation of the optical noise through the feedback controlling action of the high speed APC circuit 9.

[0094] (Second Embodiment)

[0095]FIG. 11 is a schematic view of a solid laser apparatus 200 showing the second embodiment of the present invention.

[0096] The solid laser apparatus 200 is identical in the construction to the solid laser apparatus 100 of the first embodiment, except that the microchip laser crystal 3 is eliminated.

[0097] The solid laser apparatus 200 provides the same advantages as the solid laser apparatus 100 of the first embodiment. More specifically, the combination of a lowpass filter 9 c and a highpass filter 9 d can allow any optical noise to be effectively attenuated through the feedback controlling action of a high speed APC circuit 9.

[0098] (Third Embodiment)

[0099]FIG. 12 is a schematic diagram of a solid laser apparatus 300 showing the third embodiment of the present invention.

[0100] The solid laser apparatus 300 comprises a semiconductor laser 1 for generating a laser beam, a collective lens system 2 for focusing the laser beam, a microchip laser crystal 3 provided with its crystal end faces coated for use as an optical resonator which can be excited by the focused laser beam, a nonlinear optical device 4 for receiving the laser beam from the microchip laser crystal 3 and emitting its harmonic light, an assembly of a splitter 5, an optical filter 6, and a photo diode 7 for detecting the intensity of the light emitted from the nonlinear optical device 4, a low speed APC (automatic power control) circuit 8 for releasing a control signal L to set the intensity of the beam detected by the photo diode 7 to a predetermined level, a high speed APC circuit 9 for releasing a control signal H to attenuate noise components in the beam detected by the photo diode 7 to zero, and an LD drive circuit 10 for supplying the semiconductor laser 1 with a driving current determined by the control signals L and H.

[0101] The low speed APC circuit 8 includes a signal amplifier circuit 8 b and a signal inverse amplifier circuit 8 e.

[0102] The high speed APC circuit 9 includes a coupling capacitor 9 a, a signal amplifier circuit 9 b, a phase shift circuit 9 g, and a signal inverse amplifier circuit 9 e.

[0103]FIG. 13 illustrates an exemplary circuit of the phase shift circuit 9 g.

[0104] The phase shift circuit 9 g is designed for advancing the phase of a signal by, e.g., 90 degrees in the vicinity of the relaxation oscillating frequency fk. Actually, the phase shift may be determined to cancel a delay in the whole system (including delays along the other electric circuits and transmission cables).

[0105] Meanwhile as shown in FIG. 22, the phase transmission function of the nonlinear optical device 4 and the microchip laser crystal 3 has a 90-degree delay in the vicinity of the relaxation oscillating frequency fk where the optical noise is carried.

[0106] Accordingly, a combination of the phase transmission function of the nonlinear optical device 4 and the microchip laser crystal 3 and the phase transmission function of the phase shift circuit 9 g allows the phase delay to be reduced to zero in the vicinity of the relaxation oscillating frequency fk, hence enabling the attenuation of the optical noise through the feedback controlling action of the high speed APC circuit 9.

[0107] (Fourth Embodiment)

[0108]FIG. 14 is a schematic view of a solid laser apparatus 400 showing the second embodiment of the present invention.

[0109] The solid laser apparatus 400 is identical in the construction to the solid laser apparatus 300 of the first embodiment, except that the microchip laser crystal 3 is eliminated.

[0110] The solid laser apparatus 400 provides the same advantages as the solid laser apparatus 300 of the first embodiment. In brief, the attenuation of optical noise can be carried out through the feedback controlling action of a high speed APC circuit 9.

[0111] (Fifth Embodiment)

[0112]FIG. 15 is a schematic diagram of a solid laser apparatus 500 showing the first embodiment of the present invention.

[0113] The solid laser apparatus 500 comprises a semiconductor laser 1 for generating a laser beam, a collective lens system 2 for focusing the laser beam, a microchip laser crystal 3 provided with its crystal end faces coated for use as an optical resonator which can be excited by the focused laser beam, a nonlinear optical device 4 for receiving the laser beam from the microchip laser crystal 3 and emitting its harmonic light, an assembly of a splitter 5, an optical filter 6, and a photo diode 7 for detecting the intensity of the light emitted from the nonlinear optical device 4, a low speed APC (automatic power control) circuit 8 for releasing a control signal to set the intensity of the beam detected by the photo diode 7 to a predetermined level, a high speed APC circuit 9 for releasing a control signal H to attenuate noise components in the beam detected by the photo diode 7 to zero, and an LD drive circuit 10 for supplying the semiconductor laser 1 with a driving current determined by the control signals L and H.

[0114] The low speed APC circuit 8 includes a signal amplifier circuit 8 b and a signal inverse amplifier circuit 8 e for suppressing any fluctuation in the DC level of optical output.

[0115] The high speed APC circuit 9 includes a coupling capacitor 9 a, a signal amplifier circuit 9 b, a pseudo notch filter 9 h, and a signal inverse amplifier circuit 9 e.

[0116]FIG. 16 illustrates an exemplary circuit of the pseudo notch filter 9 h.

[0117]FIG. 17 illustrates a profile of the gain transmission characteristic in the pseudo notch filter 9 h.

[0118] The pseudo notch filter 9 h is characterized for the gain gradually decreasing towards and becoming local minimum in the vicinity of the relaxation oscillating frequency fk of the nonlinear optical device 4 and the microchip laser crystal 3. As the result, the pseudo notch filter 9 h can offset the peak of the gain transmission characteristic of the nonlinear optical device 4 and the microchip laser crystal 3, hence enabling the attenuation of optical noise in the vicinity of the relaxation oscillating frequency fk.

[0119]FIG. 18 illustrates a profile of the phase transmission characteristic in the pseudo notch filter 9 h.

[0120] The pseudo notch filter 9 h allows the phase of a signal to be inverted in the vicinity of the relaxation oscillating frequency fk or notch frequency. More particularly, the phase is −180 degrees at frequencies lower than the relaxation oscillating frequency fk and zero degree at frequencies higher than the same.

[0121] Meanwhile, the phase in the phase transmission function of the nonlinear optical device 4 and the microchip laser crystal 3 is zero degree at the frequencies lower than the relaxation oscillating frequency fk and −180 degrees at the frequencies higher than the relaxation oscillating frequency fk, as shown in FIG. 23.

[0122] Accordingly, the phase transmission function of the pseudo notch filter 9 h in combination with that of the nonlinear optical device 4 and the microchip laser crystal 3 can provide −180 degrees of the phase at the frequencies lower than the relaxation oscillating frequency fk as well as the frequencies higher than the relaxation oscillating frequency fk. In short, the inversion of the phase is eliminated in the vicinity of the relaxation oscillating frequency fk.

[0123] As the result, the attenuation of both low frequency noise and high frequency noise can successfully be carried out by the feedback controlling action of the high speed APC circuit 9.

[0124] (Sixth Embodiment)

[0125]FIG. 19 is a schematic view of a solid laser apparatus 600 showing the second embodiment of the present invention.

[0126] The solid laser apparatus 600 is identical in the construction to the solid laser apparatus 500 of the first embodiment, except that the microchip laser crystal 3 is eliminated.

[0127] The solid laser apparatus 600 provides the same advantages as the solid laser apparatus 500 of the first embodiment. More specifically, both low frequency noise and high frequency noise can effectively be attenuated by the feedback controlling action of a high speed APC circuit 9.

[0128] Industrial Applicability

[0129] The solid laser apparatus according to the present invention is allowed to be employed in such as bioengineering field and measuring field. 

What is claimed is:
 1. A solid laser apparatus having a semiconductor laser for emitting a laser beam, a microchip laser crystal arranged for being excited by the laser beam emitted from the semiconductor laser and provided with its crystal end faces coated for use as an optical resonator, a nonlinear optical device for receiving a laser beam from the microchip laser crystal and emitting its harmonic light, an optical detecting means used as a monitor for detecting the intensity of the light emitted from the nonlinear optical device, and an output control circuit for driving the semiconductor laser so as to maintain the intensity of the light at a predetermined level, wherein the output control circuit includes a lowpass filter and a highpass filter.
 2. A solid laser apparatus according to claim 1, wherein the cutoff frequency of the highpass filter is arranged higher than the cutoff frequency of the lowpass filter.
 3. A solid laser apparatus having a semiconductor laser for emitting a laser beam, a microchip laser crystal arranged for being excited by the laser beam emitted from the semiconductor laser and provided with its crystal end faces coated for use as an optical resonator, a nonlinear optical device for receiving the laser beam from the microchip laser crystal and emitting its harmonic light, an optical detecting means used as a monitor for detecting the intensity of the light emitted from the nonlinear optical device, and an output control circuit for driving the semiconductor laser so as to maintain the intensity of the light at a predetermined level, wherein the output control circuit includes a bandpass filter of which the center frequency is arranged higher than the relaxation oscillating frequency of the solid laser apparatus.
 4. A solid laser apparatus having a semiconductor laser for emitting a laser beam, a nonlinear optical device for receiving a laser beam from the semiconductor laser and emitting its harmonic light, an optical detecting means used as a monitor for detecting the intensity of the light emitted from the nonlinear optical device, and an output control circuit for driving the semiconductor laser so as to maintain the intensity of the light at a predetermined level, wherein the output control circuit includes a lowpass filter and a highpass filter.
 5. A solid laser apparatus according to claim 4, wherein the cutoff frequency of the highpass filter is arranged higher than the cutoff frequency of the lowpass filter.
 6. A solid laser apparatus having a semiconductor laser for emitting a laser beam, a nonlinear optical device for receiving a laser beam from the semiconductor laser and emitting its harmonic light, an optical detecting means used as a monitor for detecting the intensity of the light emitted from the nonlinear optical device, and an output control circuit for driving the semiconductor laser so as to maintain the intensity of the light at a predetermined level, wherein the output control circuit includes a bandpass filter of which the center frequency is arranged higher than the relaxation oscillating frequency of the solid laser apparatus.
 7. A solid laser apparatus having a semiconductor laser for emitting a laser beam, a microchip laser crystal arranged for being excited by the laser beam emitted from the semiconductor laser and provided with its crystal end faces coated for use as an optical resonator, a nonlinear optical device for receiving a laser beam from the microchip laser crystal and emitting its harmonic light, an optical detecting means used as a monitor for detecting the intensity of the light emitted from the nonlinear optical device, and an output control circuit for driving the semiconductor laser so as to maintain the intensity of the light at a predetermined level, wherein the output control circuit includes a phase shift circuit for advancing the phase of a signal in the vicinity of the relaxation oscillating frequency of the solid laser apparatus.
 8. A solid laser apparatus having a semiconductor laser for emitting a laser beam, a nonlinear optical device for receiving a laser beam from the semiconductor laser and emitting its harmonic light, an optical detecting means used as a monitor for detecting the intensity of the light emitted from the nonlinear optical device, and an output control circuit for driving the semiconductor laser so as to maintain the intensity of the light at a predetermined level, wherein the output control circuit includes a phase shift circuit for advancing the phase of a signal in the vicinity of the relaxation oscillating frequency of the solid laser apparatus.
 9. A solid laser apparatus having a semiconductor laser for emitting a laser beam, a microchip laser crystal arranged for being excited by the laser beam emitted from the semiconductor laser and provided with its crystal end faces coated for use as an optical resonator, a nonlinear optical device for receiving a laser beam from the microchip laser crystal and emitting its harmonic light, an optical detecting means used as a monitor for detecting the intensity of the light emitted from the nonlinear optical device, and an output control circuit for driving the semiconductor laser so as to maintain the intensity of the light at a predetermined level, wherein the output control circuit includes a pseudo notch filter of which gain is arranged to have a local minimum at the relaxation oscillating frequency and not zero at the notch frequency.
 10. A solid laser apparatus having a semiconductor laser for emitting a laser beam, a nonlinear optical device for receiving a laser beam from the semiconductor laser and emitting its harmonic light, an optical detecting means used as a monitor for detecting the intensity of the light emitted from the nonlinear optical device, and an output control circuit for driving the semiconductor laser so as to maintain the intensity of the light at a predetermined level, wherein the output control circuit includes a pseudo notch filter of which gain is arranged to have a local minimum at the relaxation oscillating frequency and not zero at the notch frequency. 